Thermoacoustic engines are known as a device for recovering heat (exhaust heat) of a heat source. A typical example of such known thermoacoustic engines is disclosed in Japanese Patent Application Laid-Open Publication (JP-A) No. 2000-88378. The disclosed thermoacoustic engine includes a stack and a heat storage unit that are embedded in a gas-filled looped tube, and a hot-side heat exchanger and a cold-side heat exchanger that are disposed on opposite sides of each of the stack and the heat storage unit.
In order to recover exhaust heat from a heat source, the hot-side heat exchanger associated with the stack is heated with heat supplied from the heat source, while the cold-side heat exchanger associated with the stack and the cold-side heat exchanger associated with the heat storage unit are cooled. Due to a temperature gradient created across the stack, the gas in the stack undergoes self-exited oscillation and the stack induces a sound wave. The sound wave propagates through the gas to the heat storage unit, thereby heating the hot-side heat exchanger associated with the heat storage unit. The exhaust heat of the heat source is thus recovered.
Since the thermoacoustic engine disclosed in JP 2000-88378 A has only one stack in the looped tube, the efficiency of converting heat energy (exhaust heat of the heat source) to acoustic power is relatively low.
As the heat source for the thermoacoustic engine, various exhaust heats, including engine exhaust heat and boiler exhaust heat, can be used. However, such exhaust heats are not constant in temperature. Furthermore, if the exhaust heat is near room temperature, efficient recovery of the exhaust heat by the conventional thermoacoustic engine is practically impossible.
It is an object of the present invention to provide a thermoacoustic engine which is capable of recovering heat with high efficiencies even when temperature of heat from a heat source is relatively low.